| BACKGROUND |
| This document does not try to explain the concept of Celestial Navigation. One good source of information on the subject may be found on the Internet. The link to it is: A Short Guide to Celestial Navigation http://home.t-online.de/home/h.umland/ This simulation is based on a RAF bubble sextant, of WW2 vintage, called the Mark IX. There were several versions of this bubble sextant made during the war, adding mechanical averaging, first of six shots, then a running 1 or 2 minute average of the position. This simulation approximates a one minute average. The real sextant has several filters for shading the sun. Our simulation only has one filter. The real sextant jitters both the x and y axis, our simulation only jitters in the y axis, the one which directly affects the accuracy of the measurement. The simulation includes a worksheet on which you specify your assumed position, and the star's assumed azimuth and elevation. As with the real sextant, the simulation sets the "mirror" to where you think the star elevation will be, so when you first look at the display, you are seeing on the azimuth and elevation where you think the star will be. As with a real sextant, if you have made a large error in your assumed position, you won't find the star, and the assumed position must be changed. As with the real sextant you may set the bubble size, which should be between 1/3 and 1/2 the distance between the lines seen in the field of view. The sun and bright stars should be aligned in the center of the bubble, and faint stars may be placed either centrally inside the bubble or level with the bubble close to it at one side, at the preference of the observer. All observations should be made with the object and the bubble between the two vertical lines seen in the field of view. The real movement or undulating wander of the star and bubble in the field of view is caused by the sextant movement. The star doesn't move. This simulation has the star moving, as if the eye and the sextant have no relative movement. The real bubble movement is also caused by the movement of the sextant. The simulation has the bubble moving, as the integral of the star movement damped by the inertia of the alcohol forming the bubble. Considerable effort went into making the wander look and feel "normal". This wander is based on data in the RAF navigation manual depicting the typical airplane's movement during a shot. Old time autopilots, and perhaps real pilots also, had a cyclic motion, similar to a sine wave, that adjusted the pitch, yaw, and roll of the plane back to the correct position to hold constant direction and altitude. The real Mark IX sextant displays the elevation angle as the sum of four different displays. The first is a detented 10 degree step from -10 to +80 degrees. Next is a rotating scale graduated in one minute increments from 0 to 60 minutes. Third is also a rotating scale graduated in degrees from 0 to 10, and finally a selectable 0 or 5 degree mechanical offset lever, so you don't have to operate near one end of the 0 to 10 degree scale rotation. Originally a goal was to simulate these four displays photo-realistically. It just seemed too busy and unnecessarily complex. The simulation simplifies this to a digital display of degrees and minutes controlled by up/down hot-buttons. So the basic accuracy of the simulated shot is limited to the display precision of 5 nm. The worst case is thus a 2.5 nm error, plus or minus. On average you have much less error than that, discounting the jitter. After taking the shot, the real sextant only displays the measured elevation angle as described above. With the simulation, the distance is also displayed on the worksheet, to simplify that calculation a little. This distance, in Nautical Miles, is the distance, on the azimuth to the star, that you need to move the Line of Position. You still need to manually plot the distance, and Line of Position, on a paper chart. |